Implement storage::BitVec

[Robbepop]

Storage Bitset

The pdsl_core is currently missing an efficient and elegant way to handle bit sets.
Using a storage::Vec<bool> is very inefficient, since it would require storing every single bit in a different storage cell and using storage::Vec<[u8; 32]> (or similar) would require implementation effort from the user.

The storage::Bitset should provide the most basic functionality for sets of bits and should be very efficient in the way that it reduces contract storage access to a minimum while keeping up with overall performance.

Bits stored in the storage::Bitset should be bundled - for example as [u8; 32]. This allows efficient contract storage access and lots of cache efficiency on the memory side.

A storage::Bitset should allow iteration and the following APIs:

• symmetric_difference(&mut self, other: &Bitset) *
• difference(&mut self, other: &Bitset) *
• union(&mut self, other: &Bitset) *
• intersection(&mut self, other: &Bitset) *

(*) Means that this API is not final.

• is_disjoint(&self, other: &Bitset) -> bool
• is_subset(&self, other: &Bitset) -> bool
• is_superset(&self, other: &Bitset) -> bool
• len(&self) -> u32
• is_empty(&self) -> bool
• contains(&self, n: i32) -> bool

Depending on the concrete bit set implementation we could either have one of the following APIs:

• storage::Bitset based on storage::Vec
  • push
  • pop
• storage::Bitset based on storage::HashMap
  • insert
  • remove

We can also think about implementing both of the above versions.

[Robbepop]

Commit ff1421c implements an initial version of a BitVec that is more similar to a vector than to a set of bits. However, this is the first step taken and could also potentially be used for many use bases.

[Robbepop]

Commit 7155c9b fixes a bug that first_set_position returned Some bit if the last set bit was just popped right before.

[Robbepop]

Closing this since its fundamentals are implemented.